1. Field of the Invention
The present invention relates to a polymer dispersed liquid crystal display device and a method of manufacturing the same.
2. Description of the Related Art
A polymer dispersed liquid crystal is obtained by dispersing a nematic liquid crystal in a transparent polymeric material. In a liquid crystal display device using the polymer dispersed liquid crystal, a polymer dispersed liquid crystal layer is provided between a pair of transparent substrates on which transparent electrodes are formed.
A transmission type liquid crystal display device having the polymer dispersed liquid crystal layer is driven when an electric field is applied across the electrodes of the two substrates, so that display is performed. The molecules of the liquid crystal in the polymer dispersed liquid crystal layer are directed in various directions when an electric field is not applied. In this state, light transmitted through the polymer dispersed liquid crystal layer is scattered by the light-scattering function caused by the liquid crystal and by the light-scattering function that occurs at the interface between the liquid crystal and the polymeric material. Hence, a portion of the display device to which the electric field is not applied is set in a dark display state which is close to black.
When an electric field higher than the threshold voltage of the liquid crystal is applied across the electrodes, the molecules of the liquid crystal are uniformly aligned so as to be perpendicular to the surfaces of the substrates. Then, the transmitted light is transmitted through the polymer dispersed liquid crystal layer without substantially being subjected to the light-scattering function. Hence, a portion of the display device to which the electric field is applied is set in a bright display state.
Since the polymer dispersed liquid crystal display device realizes display by controlling the amount of transmitted light and the amount of scattered light, it does not need a polarizing plate. Therefore, this polymer dispersed liquid crystal display device can obtain brighter display than that of a conventional TN liquid crystal display device.
In recent years, it is proposed to adapt the polymer dispersed liquid crystal display device having the above advantages to an active matrix type liquid crystal display device which is driven by a thin film transistor (to be referred to as a TFT hereinafter). Although the TFT-driven polymer dispersed liquid crystal display device can be driven in a high time-division manner and can obtain a bright and high-definition dot display image, it is difficult to obtain a high contrast. This is because at the portion among adjacent pixels to which an electric field is not applied, light is always scattered by the polymer dispersed liquid crystal, and the scattered light leaks to the outside.
The liquid crystal display device is manufactured in accordance with a method wherein the device is assembled by bonding a pair of substrates through a frame-shaped seal member, a solution obtained by mixing a monomer of a photo-setting polymeric material and a liquid crystal is filled in the device, and thereafter, the monomer of the polymeric material is polymerized by light radiation, thereby forming a polymer dispersed liquid crystal layer. If, however, the gap between the substrates of the device assembled in this manner is non-uniform, the thickness of the polymer dispersed liquid crystal layer formed between the substrates becomes non-uniform to cause non-uniformity in display. For this reason, conventionally, transparent substrate gap regulating spacers made of spherical grass particles or short glass fibers are distributed between a pair of substrates. The substrate gap is regulated by these spacers, thereby setting the thickness of the polymer dispersed liquid crystal layer substantially uniform throughout the entire layer.
However, in the conventional liquid crystal display device, light that passes through portions where the substrate gap regulating spacers are located is transmitted through the spacers and emitted without substantially being scattered. Therefore, bright spots corresponding to the shapes of the spacers appear in the dark display region which is displayed by light scattering in the polymer dispersed liquid crystal layer. The display quality is degraded by the bright spots.
Especially, in the TFT-driven polymer dispersed liquid crystal display device which is often used for small-pixel high-definition display, e.g., a video display, occurrence of bright spots as described above in the small pixel poses a serious problem.
Furthermore, although the polymer dispersed liquid crystal display device has a bright screen, it requires a high drive voltage since the liquid crystal is dispersed in the resin, so that it is hindered from being put into practical use as a display device. Therefore, it is conventionally proposed to increase the proportion of the liquid crystal in the polymer dispersed liquid crystal layer. If the proportion of the liquid crystal in the polymer dispersed liquid crystal layer is increased, the voltage drop caused by the resin component is decreased by the increase in proportion of the liquid crystal, thereby decreasing the drive voltage. According to the conventional manufacturing method, however, if the proportion of the liquid crystal in the solution mixture to be injected into the cell is increased, a polymer dispersed liquid crystal layer in which the liquid crystal is uniformly dispersed cannot be obtained, causing display non-uniformity in the polymer dispersed liquid crystal display device. This is due to the following reason.
Namely, even if the solution mixture is composed by dissolving the liquid crystal in the saturated state, when the solution mixture is injected into the cell in accordance with vacuum injection, the pressure of the mixture injected in the cell in the vacuum state is decreased to decrease the solubility of the liquid crystal. When the solubility of the liquid crystal of the mixture is decreased, an amount of liquid crystal that exceeds the solubility is separated to form in the mixture a liquid crystal mass constituted by only the gathered liquid crystal. To form the polymer dispersed liquid crystal layer, the mixture is injected into the cell, and thereafter the photo-setting resin in the mixture is subjected to photopolymerization. Hence, if a liquid crystal mass is present in the mixture, the dispersed state of the liquid crystal in the polymer dispersed liquid crystal layer becomes non-uniform, and thus the optical characteristics (transmitting and scattering characteristics) of the polymer dispersed liquid crystal layer become non-uniform, causing display non-uniformity.
In photopolymerization described above, if a smooth polymerization reaction of polymers is interfered with by the molecules of the liquid crystal, the structure of a uniform polymer dispersed liquid crystal layer cannot be obtained, and the optical characteristics become non-uniform, thereby causing display non-uniformity.